1. Field of the Invention
The present invention is related to an uninterruptible power system (UPS), and more particularly to a hybrid green uninterruptible power system which concurrently has an AC output port and a DC output port, a bi-directional converter module and a power conversion method thereof.
2. Description of Related Art
A UPS (Uninterruptible Power System) is a protection device for computer equipment, monitoring instruments, fire-fighting equipment, and medical equipment. When the supply of AC utility power is interrupted, the UPS acts as a substitute for the AC utility power to ensure the equipment/instrument does not shutdown or become damaged owing to the instantaneous power disconnection. Alternatively, when the power source becomes invalid, for example, an irregular voltage (such as over voltage and under voltage), or a transient voltage surge caused by a lightning strike, the quality of the power for supporting normal operation of the equipment/instrument is influenced. IN such cases, a UPS can help output clean AC power.
Please refer to FIG. 1 which shows a schematic view of a conventional uninterruptible power system (UPS) 10. As shown in FIG. 1, the UPS 10 includes an input port 100, a relay 102, a battery charger module 106, a secondary battery 108, a boost module 110, an inverter module 112, a monitor module 120, and an output port 104. The input port 100 is coupled to the utility power network for receiving an AC utility power, and the output port 104 outputs an AC power to the external load.
The UPS 10 in FIG. 1 is called an off-line UPS or a passive standby UPS. It is characterized by a relay 100 that switches the power supply source. When the AC utility power operates normally, the relay 100 stays in an On/Close state so that AC utility power is directly provided to the load through the output port 102. When the AC utility power becomes invalid, the relay 100 is switched to an Off/Open state, so that an AC power produced by the UPS 10 is provided to the load. In FIG. 1, the transmission paths A and B respectively represent the power transmission paths when the AC utility power is normal and invalid.
The input port 100 is connected to the utility power network for receiving AC utility power. The monitor module 120 is coupled to the input port 100 for detecting whether the electrical characteristics of the inputted AC utility power is normal or abnormal so as to control the operation of the UPS 10. When the electrical characteristics of the AC utility power are normal, the relay 100 stays in the On/Close state, and then a battery charger module 106 converts the AC utility power into DC power and also steps down the DC power, so as to charge the secondary battery 108. Once the AC utility becomes invalid, the relay 100 is switched to the Off/Open state and the power of the secondary battery 108 is released. The boost module 110 also boosts the output DC power released from the secondary battery 108. Next, the inverter module 112 inverts the high-voltage DC power outputted by the boost module 110 into AC power with the utility power frequency by use of an internal bridge circuit so as to supply AC power to the load through the output port 104.
Apart from the off-line UPS described above, another commonly used UPS is an on-line UPS. The internal connection among modules of an on-line UPS is different from that of an off-line UPS. In an on-line UPS AC utility power is converted into DC power which the DC power is used to charge the secondary battery and also be inverted into AC power with the utility power frequency by an inverter module. Once the AC utility power is invalid, the DC power stored in the secondary battery is released and the released DC power will be boosted by the boost module and further transformed into AC power by the inverter module.
According to the above description, it is realized that in a UPS, the boost module, which is used for converting high energy voltage, only works when the AC utility power becomes invalid. In particular, in off-line UPS, the boost module and the inverter module both remain idle as the AC utility power operates normally. Obviously this is a waste of resources. Furthermore, there are many digital devices, including notebooks, mobile phones, and portable DVD players, which operate with a DC power supply. Once the normal power supply is interrupted to these devices, AC power outputted by the UPS has to be converted into DC power by use of the voltage converter. Therefore, the power loss not only occurs as the power outputted from the secondary battery is internally converted into the AC power, but also occurs as the AC power is once again converted by an external voltage converter to change back to DC power. Thereby, usually, only 50% to 60% of the power released from the secondary battery can be transmitted to the device. Obviously this is actual waste of energy and does not meet the requirement of environmental consciousness.